An Area-to-Inv Bijection Between Dyck Paths and 312-avoiding Permutations Jason Bandlow and Kendra Killpatrick Mathematics Department Colorado State University Fort Collins, Colorado bandlow@math.colostate.edu killpatr@math.colostate.edu Submitted: July 23, 2001; Accepted: December 10, 2001. MR Subject Classifications: 05A15, 05A19 Abstract The symmetric q, t-Catalan polynomial C n (q, t), which specializes to the Catalan polynomial C n (q)whent = 1, was defined by Garsia and Haiman in 1994. In 2000, Garsia and Haglund described statistics a(π)andb(π) on Dyck paths such that C n (q, t)=  π q a(π) t b(π) where the sum is over all n × n Dyck paths. Specializing t = 1 gives the Catalan polynomial C n (q) defined by Carlitz and Riordan and further studied by Carlitz. Specializing both t =1andq = 1 gives the usual Catalan number C n . The Catalan number C n is known to count the number of n × n Dyck paths and the number of 312-avoiding permutations in S n ,aswellasat least 64 other combinatorial objects. In this paper, we define a bijection between Dyck paths and 312-avoiding permutations which takes the area statistic a(π)on Dyck paths to the inversion statistic on 312-avoiding permutations. The inversion statistic can be thought of as the number of (21) patterns in a permutation σ.We give a characterization for the number of (321), (4321), ,(k ···21) patterns that occur in σ in terms of the corresponding Dyck path. 1 Introduction The polynomial C n (q, t), called the q, t-Catalan polynomial, was introduced in 1994 by Garsia and Haiman [5]. They conjectured that it is the Hilbert series of the diagonal harmonic alternates and showed that it is the coefficient of the elementary symmetric function e n in the symmetric polynomial DH n (x; q, t); the conjectured Frobenius charac- teristic of the module of diagonal harmonic polynomials. The polynomial is referred to as the q, t-Catalan polynomial because specializing t =1givestheq-Catalan polynomial the electronic journal of combinatorics 8 (2001), #R40 1 first given by Carlitz and Riordan [3] and further studied by Carlitz [2]. Specializing both q = t = 1 results in the well-known Catalan number C n = 1 n+1  2n n  . In order to give the precise definition of C n (q, t), we must first introduce some notation. A sequence µ =(µ 1 ,µ 2 , ,µ k ) is said to be a partition of n if µ 1 ≥ µ 2 ≥···≥µ k > 0and µ 1 + µ 2 + ···+ µ k = n. A partition µ may be described pictorially by it’s Ferrers diagram, an array of n dots into k left-justified rows with row i containing µ i dots for 1 ≤ i ≤ k. Using the Ferrers diagram we can define the transpose of a partition µ, denoted µ T ,to be the partition whose ith row (numbered from the top) is the length of the ith column in the Ferrers diagram of µ. The symbols l and l  are used to represent the leg and coleg of a cell: the number of cells strictly below and strictly above a given cell, respectively. Similarly, a and a  represent the arm and coarm of a cell: the number of cells strictly to the right and strictly to the left of a given cell, respectively. For example, for the labelled cell s in the diagram below, a =5,a  =4,l =3,andl  =2. The precise definition of C n (q, t) is given as follows: C n (q, t)=  µn t 2Σl q 2Σa (1 − t)(1 − q)  (0,0) (1 − q a  t l  )  q a  t l   (q a − t l+1 )(t l − q a+1 ) The summations and products within the µ th summand are over all cells in the Ferrers diagram of a given partition, and the symbol Π (0,0) is used to represent the product over all cells but the upper left corner. The q, t-Catalan polynomial is symmetric in q and t; i.e, C n (q, t)=C n (t, q). To see this, note that for every µ  n, µ T is also a partition of n. We can see that the summand corresponding to µ in C n (q, t) will equal the summand corresponding to µ T in C n (t, q) by observing the relationships between a, l, a  ,andl  in µ and µ T . Given a cell s in a partition µ, the arm length of s in µ equals the leg length of the corresponding cell s  in µ T , and vice-versa. Similarly, the lengths of the coarm and coleg of s and s  are also interchanged. This can be seen in the diagram below, which shows the transpose of the first diagram, and the corresponding cell, s  . Note that here, l =5,l  =4,a =3,and a  =2. the electronic journal of combinatorics 8 (2001), #R40 2 Though symmetry follows directly from the definition of the q, t-Catalan polynomial, it is less obvious that this polynomial has positive integer coefficients. To prove this, Garsia and Haiman conjectured that there exist statistics a(π)andb(π)onn × n Dyck paths, D n , such that C n (q, t)=  π∈D n q a(π) t b(π) . (A complete description of Dyck paths follows in Section 2.) For a given path π,thestatistica(π) is the number of full squares which lie below the path and completely above the line y = x [4]. To compute b(π), we first construct a second Dyck path from π called β(π); then b(π) is the sum of the x-coordinates of the points where this second path touches the line y = x, excluding the points (0, 0) and (n, n). (See Section 3 for a more complete description of b(π).) The b(π) statistic was first conjectured by Haglund [7] and then proved to be correct by Garsia and Haglund [5]. Since C n (q, t)=C n (t, q), there must exist a bijection from D n to D n that maps π to π  , such that a(π)=b(π  )andb(π)=a(π  ). It is an open problem to define such a bijection constructively. A possible approach to find such a bijection is to make use of some of the 66 other known descriptions of the Catalan numbers [11]. More work has been done on the development of statistics for some of these objects than others. In particular, many statistics have been developed for permutations. While investigat- ing permutations that could be sorted on a single pass through a stack, Knuth [8] showed that (312)-avoiding permutations satisfy the Catalan recurrence. The inv statistic, or in- version statistic, is defined as the number of “inversions” in a permutation, or alternately as n(21): the number of patterns of the form (21) in a permutation. F¨urlinger and Hof- bauer [4] proved that the q-Catalan polynomial is the generating function for inversions on (312)-avoiding permutations. The main result of this paper is to give a bijection from Dyck paths to (312)-avoiding permutations that sends the area statistic on Dyck paths to the inv statistic on (312)- avoiding permutations. In addition, we will classify the occurrence of permutation pat- terns n(321), n(4321), , n(k ···21) in terms of Dyck paths. Since the a(π)andb(π) statistics are equidistributed on Dyck paths, the hope is to find another statistic on permutations which, when restricted to (312)-avoiding permutations, gives the corresponding b(π) statistic under our bijection. An examination of the statistics known to be equidistributed with inv, however, has not yet yielded any such result. the electronic journal of combinatorics 8 (2001), #R40 3 In section 2, we give the necessary definitions and background for this paper. Section 3 contains the construction of our bijection and the proof that it sends the area statistic on Dyck paths to the inv statistic on (312)-avoiding permutations. Section 4 gives our characterization of n(321), n(4321), , n(k ···21), and Section 5 discusses some open questions. 2 Background and Definitions The Catalan sequence is the sequence {C n } ∞ n=0 = {1, 1, 2, 5, 14, 42, } where C n = 1 n +1  2n n  . C n is called the nth Catalan number. The Catalan numbers have been shown to count certain properties on more than 66 different combinatorial objects (see Stanley [11] pg. 219, Exercise 6.19 for a complete list). The objects of use to us in this paper will be certain lattice paths called Dyck paths and certain permutations called 312-avoiding per- mutations. A Dyck path is a lattice path in Z 2 from (0, 0) to (n, n) consisting of only steps in the positive x direction (EAST steps) and steps in the positive y direction (NORTH steps) such that there are no points (x, y) on the path for which x>y. In other words, a Dyck path is a path from (0, 0) to (n, n) consisting only of NORTH and EAST steps that never goes below the diagonal. Let D n denote the set of Dyck paths from (0, 0) to (n, n). For example, D 3 consists of the following paths: The Catalan number C n is known to count the number of Dyck paths from (0, 0) to (n, n), thus C 3 =5. Thelength of a Dyck path is the number of NORTH steps in the path, thus a Dyck path π ∈ D n has length n. Let S n denote the symmetric group on [n]={1, 2, ,n}.Atransposition s i =(i, i+1) is a function from S n to S n which interchanges the numbers in the ith and (i+1)st position in a permutation. For example, s 4 (512867394) = 512687394. It is well-known that every permutation σ in S n can be represented as a sequence of trans- positions s i 1 s i 2 s i k which, when applied from right to left to the identity permutation 123 ···n,resultsinσ. This representation is not necessarily unique. For example, 321 can be written as both s 1 s 2 s 1 and s 2 s 1 s 2 . We will describe one method for writing a permutation as such a product of transpositions in the following section. the electronic journal of combinatorics 8 (2001), #R40 4 A 312-avoiding permutation π ∈ S n is a permutation π = π 1 π 2 ···π n containing no triple π i π j π k with i<j<ksuch that π i >π k >π j . For example, π = 2143 is a (312)-avoiding permutation in S 4 while σ = 4213 is not. Let S n (312) denote the set of all 312-avoiding permutations in S n and let A n (312) = |S n (312)|. In [8], Knuth proved that, for every R ∈ S 3 , A n (R)= 1 n +1  2n n  = C n . In particular, if R = (312), this proves that C n equals the number of 312-avoiding per- mutations. In addition to having an explicit formula, the Catalan numbers are known to satisfy the recurrence C n = n  i=1 C i−1 C n−i . This can easily be visualized using the Dyck paths. Given a Dyck path from (0, 0) to (n, n), label the diagonal points in Z 2 as a i =(i, i) for 1 ≤ i ≤ n.Let A i = {Dyck paths from (0, 0) to (n, n) that first touch the diagonal at a i }. In other words, A i is the set of paths for which i is the smallest integer such that (i, i)isa point on the path. Then clearly C n =  n i=1 |A i |. It remains to show that |A i | = C i−1 C n−i . If a path first touches the diagonal at (i, i), the path must go from (0, 1) to (i − 1,i) without touching the diagonal points (1, 1), (2, 2), ,(i − 1,i− 1). The number of such paths is C i−1 . Once the path touches (i, i)itmustthencontinueto(n, n) without going below or to the right of the diagonal. The number of such paths is C n−i .Thus |A i | = C i−1 C n−i and therefore C n = n  i=1 |A i | = n  i=1 C i−1 C n−i . For example, if n =10andi = 3, then any path in A 3 must go from (0, 0) to (0, 1), then take some path from (0, 1) to (2, 3) without touching (1, 1) or (2, 2). Since the chosen path is in A 3 , it must then go from (2, 3) to (3, 3) and then it can take any valid Dyck path from (3, 3) to (10, 10). One example of such a path is: the electronic journal of combinatorics 8 (2001), #R40 5 A statistic on a permutation, Dyck path, or other combinatorial object counts some property about that object. The inversion statistic on a permutation σ ∈ S n is defined by inv(σ)=  1≤i<j≤n σ i >σ j 1. For example, if σ = 743216598, then inv(σ) = 14 since each of the pairs (21), (31), (41), (71), (32), (42), (72), (43), (73), (74), (65), (75), (76), and (98) contributes 1 to the sum. The generating function for the inversion statistic on S n is given by  σ∈S n q inv(σ) . Two different statistics on a class of objects are said to be equidistributed if they have the same generating function on that class of objects. A statistic on permutations is called Mahonian if it is equidistributed with the inv statistic on permutations in S n . One well- known Mahonian statistic is the major index, written maj(σ), first given by MacMahon [10]. The major index is defined in terms of descents in a permutation. A descent in a permutation σ = σ 1 σ 2 σ n is a position where σ i >σ i+1 . For example, σ = 7136254 has 3 descents. The major index is defined as the sum of the positions of the descents of σ, i.e. maj(σ)=  σ i >σ i+1 i. For the previous permutation σ, maj(σ)=1+4+6=11. In addition to defining statistics on permutations, we can define statistics on Dyck paths. Given a Dyck path π ∈ D n the area statistic, a(π), is the number of squares that lie below the path and completely above the diagonal. For example, given the following Dyck path the squares counted by the area statistic are shaded, giving an area statistic of 13. the electronic journal of combinatorics 8 (2001), #R40 6 The generating function for the area statistic on Dyck paths π ∈ D n ,  π∈D n q a(π) = C n (q), is the q-Catalan polynomial [4]. Specializing q =1intheq-Catalan polynomial gives the usual Catalan number C n .F¨urlinger and Hofbauer [4] showed that C n (q)= n  i=1 q i−1 C i−1 (q)C n−i (q). To visualize this recurrence, use notation similar to our explanation of the recurrence for the Catalan numbers. Let A i (q)=  π∈A i q a(π) . Clearly, C n (q)= n  i=1 A i (q). Then to understand the q-Catalan recurrence, it is necessary to understand why A i (q)=q i−1 C i−1 (q)C n−i (q). Since a path in A i first touches the diagonal at (i, i), it must go from (0, 1) to (i − 1,i) without touching the diagonal points (1, 1), (2, 2), ,(i−1,i−1). These number of such paths has been shown to be C i−1 and thus have weight C i−1 (q). To these paths, we must add the i − 1 squares just above the diagonal from (0, 0) to (i, i). Thus the part of the paths from (0, 0) to (i, i)inA i give us a weight of q i−1 C i−1 (q). From (i, i), the paths must then continue on to (n, n) without going below the diagonal. These paths have weight C n−i (q), giving us a total weight of A i (q)=q i−1 C i−1 (q)C n−i (q). Using the same example of a path in A 3 as previously, the additional 2 squares giving the weight q 2 are shaded in black: the electronic journal of combinatorics 8 (2001), #R40 7 Returning to the q,t-Catalan polynomial of Garsia and Haiman [6], these authors showed that C n (q, 1) = C n (q). In addition, they conjectured the existence of a statistic b(π)onDyckpathsπ ∈ D n such that C n (q, t)=  π∈D n q a(π) t b(π) . Haglund [7] conjectured that b(π) was given by a statistic he called maj(β(π)). This conjecture was recently proved by Garsia and Haglund [5]. To determine the statistic maj(β(π)) for π ∈ D n , one first obtains the path β(π) which can be thought of as a “billiard ball” path. To obtain this path from π ∈ D n ,first imagine shooting a ball straight WEST from (n, n) and just below the path until reaching a vertical step in π. Reflect the path of the ball directly SOUTH from this point until reaching the diagonal. At the diagonal, reflect the path directly WEST and still slightly under the path π until reaching another vertical step in π, upon which the path is again reflected SOUTH until reaching the diagonal. Continue in this manner until reaching the point (0, 0). Label the diagonal points by (i, i)=n − i for 1 ≤ i ≤ n − 1. Then b(π)=maj(β(π)) is the sum of the labels where the path β(π) touches the diagonal (not including (n, n)or (0, 0)). For example, the bold line denotes the path π ∈ D n and the dashed line denotes the path β(π) in the diagram below. the electronic journal of combinatorics 8 (2001), #R40 8 For this path π, b(π)=maj(β(π))=2+6+7+9=24. By the symmetry of C n (q, t), as explained in the Introduction section, C n (q, t)=C n (t, q) so C n (q)=C n (q, 1) = C n (1,q)=C n (1,t)=C n (t, 1) = C n (t). Thus  π∈D n q a(π) =  π∈D n t b(π) . While a bijection between Dyck paths is known [9] that sends a Dyck path π 1 to a Dyck path π 2 such that b(π 1 )=a(π 2 ), this bijection does not have the property that a(π 1 )=b(π 2 ). Finding such a bijection is an interesting open problem and would give a combinatorial proof of the symmetry of the q, t-Catalan polynomial. 3 Bijection Between Dyck Paths and 312-avoiding Permutations Before stating and proving our main theorem, we will describe a well-defined method for writing a permutation σ ∈ S n (312) as a product of adjacent transpositions s i . Let σ ∈ S n (312). Write σ as a product of adjacent transpositions s i by first determining a specific sequence of adjacent transpositions which, when applied to σ, will give the identity permutation. Then σ can be represented by the inverse of this sequence of transpositions. To determine the specific sequence of adjacent transpositions, suppose n is in position i in σ.Thens n−1 s n−2 ···s i+1 s i (applied right to left) moves the n to position n and leaves the relative order of the numbers 1 through n − 1 unchanged. Now locate n − 1 in the resulting permutation. Suppose n − 1 is in position j. Then the sequence s n−2 s n−3 ···s j+1 s j moves the n − 1topositionn − 1. Continuing in this manner will give the identity permutation. Then σ can be represented as the inverse of this sequence of transpositions. Since s 2 i = id then s −1 i = s i so the inverse of this sequence of transpositions is the same sequence written in reverse order. Thus σ is represented by a product of the electronic journal of combinatorics 8 (2001), #R40 9 adjacent transpositions s i whose subscripts form a series of increasing subsequences, i.e., σ = σ 1 σ 2 ···σ j with j ≤ n such that each σ i is a product of adjacent transpositions whose subscripts are strictly increasing. In this representation, j is the minimum number of such subsequences. For example, let σ =23168795104. Then s 9 moves the 10 to the last position, giving s 9 (σ)=23168795410. Next s 8 s 7 moves the 9 to the 9th position, s 7 s 6 s 5 moves the 8 to the 8th position, s 6 s 5 moves the 7 to the 7th position, s 5 s 4 moves the 6 to the 6th position, s 4 moves the 5 to the 5th position, the 4 is already in the 4th position, s 2 moves the 3 to the 3rd position, and s 1 moves the 2 to the 2nd position. Then σ can be represented as the inverse of this sequence of transpositions, so σ = s 9 /s 7 s 8 /s 5 s 6 s 7 /s 5 s 6 /s 4 s 5 /s 4 /s 2 /s 1 . The symbol / has been added above only as a delimiter for the sake of readability. In this example, σ = σ 1 σ 2 ···σ 8 where σ 1 = s 9 , σ 2 = s 7 s 8 , σ 3 = s 5 s 6 s 7 , σ 4 = s 5 s 6 , σ 5 = s 4 s 5 , σ 6 = s 4 , σ 7 = s 2 ,andσ 8 = s 1 . Now we describe a function f : S n (312) → D n . Let σ = σ 1 σ 2 ···σ k ,whereeachσ i is a subsequence of adjacent transpositions with increasing subscripts, using the method described. For each i,ifσ i has length l and ends with s m , then shade in the squares of Z 2 in the (m +1)strowandincolumnsm through m − l +1. Then f(σ) is the Dyck path that has these shaded squares and only these shaded squares below it. Note that no Dyck path will ever have squares in the first row or nth column since all Dyck paths must start with a NORTH step and end with an EAST step. For σ = σ 1 σ 2 σ 8 = s 9 /s 7 s 8 /s 5 s 6 s 7 /s 5 s 6 /s 4 s 5 /s 4 /s 2 /s 1 , as in the previous example, then f(σ) is the following Dyck path: the electronic journal of combinatorics 8 (2001), #R40 10 [...]... and is 312-avoiding, thus adding n to the end still gives a 312-avoiding permutation Suppose there exist squares under the Dyck path π in row n and in columns j through ˆ n − 1 In order for π to be a Dyck path, there must be squares under the Dyck path in ˆ row n − 1 and columns j through n − 2 If π is a Dyck path of length n, then the shaded ˆ squares under the path in the first n − 1 rows form a Dyck. .. representation of σ that interchanges l and k Otherwise l would remain to the right of k and (lk) would not be an inversion in σ Theorem 1 The function f is a weight-preserving bijection from Sn (312) to Dn that maps the inversion statistic to the area statistic Proof From Lemmas 1 and 2 we may conclude that f is a bijection from 312-avoiding permutations to Dyck paths and from Lemma 3 it follows directly... interchanges n and n − 1 so n is one position to the left of n − 1 and the resulting permutation is 312-avoiding Lemma 2 If σ is a 312-avoiding permutation, then f (σ) is a Dyck path Proof Proof by induction on n If n = 1, then the only permutation is σ = 1 and the resulting path consists of one north and then one east step, which is a valid Dyck path Now assume that for every σ ∈ Sn−1 (312), f (σ) is a Dyck. .. find f −1 of a Dyck path, shade in the squares below the path Then read rows from top to bottom and within each row read left to right, writing down an sj for a shaded square in column j Lemma 1 If π is a Dyck path, then f −1 (π) is a 312-avoiding permutation Proof Proof by induction on n, the length of the Dyck path Suppose n = 1 There is only one Dyck path consisting of a north step and then an east... By symmetric, we mean that if there are k columns between a11 and a12 , then there are k rows between a11 and a21 In general, a symmetric pattern of size k is a pattern on an n × n grid of the form the electronic journal of combinatorics 8 (2001), #R40 12 with k − 1 squares in the first row and column of the pattern, k − 2 squares in the second row and column of the pattern, etc The squares in the... lie under the Dyck path f (σ) Algebraically, nσ (k · · · 21) = rows in the Dyck path f (σ) area(row) k−1 where area(row) is the number of squares under the Dyck path in that row Proof First note that if s is a square under a given Dyck path π, then every square above the diagonal that lies below or to the right of s is also a square under the Dyck path π Suppose i1 < i2 < · · · < ik−1 < m and (mik−1 ... is to translate this problem from Dyck paths to permutations, where the study of statistics is more developed This paper translates the area statistic on Dyck paths to inv on 312-avoiding permutations The hope of the authors is to find an easily defined statistic on 312-avoiding permutations which gives the b(π) statistic, and then use the ideas of known permutation bijections to prove symmetry One approach... nσ (k · · · 21) can all be described in terms of the Dyck path f (σ) and can be determined using only f (σ) by counting patterns called symmetric patterns Define a symmetric pattern of size 3 as a pattern on an n × n grid of the form such that squares a11 and a12 lie in the same row but not necessarily in adjacent columns and such that squares a11 and a21 lie in the same column but not necessarily in... example, symmetric means that if there are k columns between aij and aim , then there are k rows between aji and ami Since the patterns are symmetric, they are completely determined by the position of the squares in the top row Lemma 4 For any 312-avoiding permutation σ, the number nσ (321) equals the number of symmetric patterns of size 3 under the Dyck path f (σ) In general nσ (k · · · 21) is the number... comments and suggestions We look forward to future collaborations the electronic journal of combinatorics 8 (2001), #R40 15 References [1] BABSON, E and STEINGRIMSSON, E Generalized permutation patterns and a classification of the Mahonian statistics Sem Lothar Combin 44 (2000), Art B44b, 18pp [2] CARLITZ, L Sequences, paths, ballot numbers Fibonacci Quarterly 10 (1972), 531-549 [3] CARLITZ, L and RIORDAN, . An Area-to-Inv Bijection Between Dyck Paths and 312-avoiding Permutations Jason Bandlow and Kendra Killpatrick Mathematics Department Colorado State University Fort Collins, Colorado bandlow@math.colostate.edu killpatr@math.colostate.edu Submitted:. by Garsia and Haiman in 1994. In 2000, Garsia and Haglund described statistics a(π)andb(π) on Dyck paths such that C n (q, t)=  π q a(π) t b(π) where the sum is over all n × n Dyck paths. Specializing t. combinatorial objects. In this paper, we define a bijection between Dyck paths and 312-avoiding permutations which takes the area statistic a(π)on Dyck paths to the inversion statistic on 312-avoiding